1. Field of the Invention
The present invention relates generally to Group III nitride crystals that can be used as a Group III nitride substrate, a method of manufacturing the same, and a semiconductor device with the same.
2. Related Background Art
A Group III nitride compound semiconductor such as, for instance, gallium nitride (GaN) (hereinafter also referred to as a “Group III nitride semiconductor” or a “GaN-based semiconductor”) has been gaining attention as a material for semiconductor devices that emit blue or ultraviolet light. A laser diode (LD) that emits blue light is used for high-density optical disk devices or displays while a light emitting diode (LED) that emits blue light is used for displays, lighting, etc. It is expected to use an ultraviolet LD in the field of high technology such as, for example, biotechnology and an ultraviolet LED as, for example, an ultraviolet source for a fluorescent lamp.
Substrates of a Group III nitride semiconductor (for example, GaN) that are used for LDs or LEDs are formed, for instance, through vapor phase epitaxy. In this method, a sapphire substrate is used as the substrate and Group III nitride crystals are grown heteroepitaxially on the substrate. Generally, crystals obtained by this method have a dislocation density of about 108 cm−2 to 109 cm−2 and thus the reduction in dislocation density has been an important issue for this method. In order to resolve this issue, for example, an epitaxial lateral overgrowth (ELOG) method has been developed (see, for instance, JP11(1999)-145516A). With this method, the dislocation density can be reduced to about 105 cm−2 to 106 cm−2, but the manufacturing process is complicated, which is another problem.
On the other hand, a method of growing crystals from a liquid phase also has been studied as a manufacturing method other than the vapor phase epitaxy. However, since the equilibrium vapor pressure of nitrogen is at least 10000 atm (10000×1.013×105 Pa) at the melting point of Group III nitride single crystals such as, for instance, GaN or AlN, conventionally it has been understood that a severe condition, specifically, 8000 atm (8000×1.013×105 Pa) at 1200° C., is required for growing GaN from a liquid phase. In this connection, recently, a method has been developed in which a Na flux is used to allow GaN to be synthesized at relatively low temperature and pressure, specifically, 750° C. and 50 atm (50×1.013×105 Pa) (see, for instance, U.S. Pat. No. 5,868,837).
Recently, single crystals whose maximum crystal size is about 1.2 mm are obtained by a method in which a mixture of Ga and Na is melted in a nitrogen gas atmosphere containing ammonia at 800° C. and 50 atm (50×1.013×105 Pa), and then crystals are grown for 96 hours using the melt (see, for instance, JP2002-293696A).
Furthermore, another method has been reported in which a GaN crystal layer is formed on a sapphire substrate by a metalorganic chemical vapor deposition (MOCVD) method and then single crystals are grown by a liquid phase epitaxy (LPE) method (Jpn. J. Appl. Phys., Vol42, (2003) pp4-6).
However, the quality of the Group III nitride crystals obtained using the conventional techniques may not be sufficient. Hence, there are demands for a technique of manufacturing crystals of higher quality. In addition, the methods of manufacturing Group III nitride crystals using the conventional techniques are complicated. Furthermore, there is a problem when the Group III nitride crystals obtained by the conventional techniques are used as a substrate in the semiconductor manufacturing processes.